Yeah Steve... been around for many years but is just starting to go "mainstream." There are many technologies, the most useful for stuff that interests us is "SLA" -- stereo lithography. A part is designed with CAD software. It needs to be processed so that it is build able -- certain forms won't work because they will collapse. Then it goes to the machine which contains a vat of special goo that is cured by a laser, section by section. The part rests on a bed which drops by a tiny bit into the goo after each pass of the laser. When its all done the part emerges ready for cleanup, which typically involves cleaning up tons of tiny little support lattice.
A good machine can produce astonishingly nice parts, but like all digital technologies, there is a distinct granularity to the resolution. That is, parts with a draft angle with respect to the depth of the part exhibit a certain amount of "steppedness" which needs to be finished by sanding etc.
There are a number of companies that will "print" your parts, Shapeways being probably the biggest...
http://www.shapeways.com/Again, there are multiple technologies, each with their own applications.
